As a "budget-conscious" (wannabe-) sailor, I've become very interested in the Pearson Vanguard. I just read (in an ad) that the deck/doghouse is solid fiberglass? Is that right? Since every aspect of design/construction is a trade-off, what would be the upsides/downsides to a deck structure that has no plywood or balsa core?

The deck/doghouse is balsa cored. I did do a re-core on several parts of mine that rotted over the years. Really not that bad of a job, I just saved the cutout panels and put them back down, faired, and painted.
Compared to my currentboat the vanguard sailed extremly quickly in light winds, and had a very lively feel (for a cca full keeled boat). The only downside is the narrow hull. makes the cabin feel cramped.